Many metal components, including metal sheets, are used in vehicles, domestic appliances, office automation equipment, and the like. In many cases, such metal components are used after being obtained by imparting a metal sheet with a desired shape through pressing and then joining the metal sheet to another metal component or a plastic component by means of welding, screwing, or the like.
Meanwhile, if it is possible to achieve adhesive properties between a plastic and a surface of a metal sheet, it is possible to form a metal sheet-resin composite molded article in which a metal component and a plastic component are bonded to each other simply by pressing the metal sheet, setting the pressed metal sheet in a plastic molding die, injecting a molten plastic and then solidifying the plastic by cooling, and such a process can lead to increased efficiency of the component production process and to a reduction in weight of the component. In addition, resin materials having high affinities for metal sheets or plastics are used as methods for imparting adhesive properties between a plastic and a surface of a metal sheet, and such features have been proposed in, for example, Patent Literature 1 and 2.
Patent Literature 1 discloses forming a polar group-containing thermoplastic resin layer on a surface of a metal sheet in order to join the metal sheet to a molded body comprising a glass fiber-reinforced resin. Specifically, a maleic anhydride-modified polypropylene resin is laminated on a surface of a metal sheet by means of pressing or the like, the obtained laminate is then placed in a mold, and a glass fiber-containing polypropylene resin is then injection molded, thereby integrating the resin and the metal sheet.
However, it is thought that the adhesive strength between the polar group-containing thermoplastic resin layer and the metal sheet is insufficient, and the adhesive strength of the overall metal sheet-resin composite molded article is insufficient simply by using, as an adhesive, a resin material having a high affinity for a plastic to be subsequently joined.
Meanwhile, Patent Literature 2 discloses a resin-coated metal sheet obtained by providing a resin coating film, which contains a heat-sensitive crosslinking agent, on a metal sheet that has not been subjected to chromate treatment. In this invention, adhesive strength between the base metal sheet and the resin coating film is increased by the resin coating film having the heat-sensitive crosslinking agent and a prescribed film strength, but because bonding to a solid adherend (for example, a wood veneer, a plastic sheet, a rubber sheet, a fabric, or the like) is effected by using high temperature heating during baking, this invention is not suitable for cases in which a metal sheet is joined to a molten plastic during injection molding. This is because the period during which a metal sheet and a molten plastic are in contact with each other at a high temperature is extremely short during injection molding, and is not long enough for the heat-sensitive crosslinking agent to undergo a reaction/crosslinking. By further increasing the temperature of the plastic during the injection molding in order to facilitate a reaction of the heat-sensitive crosslinking agent, a longer period of time is required to cool and solidify the plastic, meaning that the advantage of being able to injection mold highly efficiently in a short period of time is lost.